The research within this thesis focuses on the behaviour of glass-forming systems as they are cooled towards the glass transition. In particular, the origins of fragile behaviour are examined. The study of glasses and supercooled liquids involves careful consideration of issues such as equilibrium, ergodicity and metastability. Simulations of the binary Lennard-Jones model glass former explore the diagnosis of broken ergodicity using an energy fluctuation metric and this approach is used to consider ergodic and short-time nonergodic behaviour of the diffusion constant. An underlying Arrhenius temperature dependence of the diffusion constant can be extracted from fragile, super-Arrhenius diffusion. This Arrhenius diffusion can be related to the true super-Arrhenius behaviour by a correction factor that depends on the average angle between atomic displacements in successive time intervals. This correction factor accounts for the fact that on average successive displacements are negatively correlated. This negative correlation can be linked directly with the higher apparent activation energy for diffusion in fragile glass formers at lower temperature. Using information from the potential energy surface, the process of an atom exiting its cage of nearest neighbours, a cage-break, can be explored. The assignment of such an event as a fundamental step for diffusive behaviour can be justified by accurate calculations of diffusion constants. Negative correlation is seen in the direct reversals of a large proportion of the cage-breaking events. Productive cage-breaks, i.e. cage-breaks that are not reversed, can provide a definition of superstructures (megabasins or metabasins) in the energy landscape.

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This work has addressed the gas transport rate and mechanism through plasma sprayed coatings. This is of particular interest for thermal barrier coatings (TBCs) and solid oxide fuel cell electrodes. Deposits of ZrO<SUB>2</SUB> - 8 wt% Y<SUB>2</SUB>O<SUB>3</SUB>, ZrO<SUB>2</SUB> - 14 wt% Y<SUB>2</SUB>O<SUB>3</SUB> and A1<SUB>2</SUB>O<SUB>3</SUB> have been plasma sprayed under varying conditions. Microstructural studies and density measurements have been carried out to characterise the porosity content and microcrack distribution. Crystallographic phase analysis and Young's Modulus measurements have also carried out. The gas permeability of each specimen has been measured at temperatures up to 600°C. These measurements involve a thin disc of the coating being sealed over a ceramic tube. A mass flow controller was used to set a constant gas flow rate, and the resulting pressure difference across the coating was measured once steady state had been reached. D'Arcy's Law was then used to determine the specific permeability of each specimen. Measurements were carried out on the coatings using hydrogen, oxygen and nitrogen as the permeating gases. Values of the specific permeability were of the order of 10<SUP>-16</SUP>m<SUP>2</SUP> for zirconia coatings and 10<SUP>-17</SUP> m<SUP>2</SUP> for alumina coatings. These results were correlated with microstructural observations via a simple analytical model for gas permeation, based on Percolation Theory. In this model, large pores were treated as isolated cavities with connecting microcracks, predicting a high sensitivity to the density and connectivity of microcracks. Good agreement was obtained between theory and experiment in terms of the magnitude of the permeability. The oxygen flux through the top coat of a TBC has been calculated from the permeability of the coating. Published values of the diffusion coefficient have been used to calculate the oxygen flux by diffusion. These 2 transport mechanisms have been compared and gas permeation has been found to dominate over diffusion at the operating temperatures of a TBC. Oxide growth at the bond coat / top coat interface of a TBC has been shown to be controlled by diffusion through the oxide layer.

Pyrochlores (<i>A=2_B2_O7_,Fd3m</i>) have been proposed as candidate phases for the immobilisation of actinides from high level nuclear waste. They may form part of a multi-barrier approach to final waste disposition in an underground repository. Titanate and zirconate pyrochlores are particularly interesting; titanates show high chemical durability, whilst some zirconate pyrochlores display very high resistance to structural damage by radiation. To fully evaluate the likely performance of a pyrochlore-based waste-form, a comprehensive understanding of its structure and behaviour must be developed. Related phases, which might exsolve, must also be characterised, to test whether their presence will have a deleterious effect. Various zirconate and titanate oxides including (La_1-xNd_x)₂Zr₂O_7,Nd₂(Zr_1-xTi_x)₂O₇, La₂(Zr_1-xTi_x)₂O₇ and (Y_1-xLa_x)₂Ti₂O₇ were synthesized and characterised by X-ray and neutron powder diffraction, electron microscopy and electron probe micro-analysis. Rare earth elements acted as non-radioactive analogues for actinide behaviour. The extent of solid solubility and factors governing exsolution were explored. The conductivity of Nd₂(Zr_1-xTi_x)₂O₇ ceramics was measured using impedance spectroscopy. Pyrochlores find application in a wide variety of technologies, including those which exploit the electrical properties of the pyrochlore structure. The effect of Ti substitution in Nd₂Zr₂O₇ was studied and compared to the conductivity of monoclinic Nd₂Ti₂O₇. The correlation between ionic mobility and critical radiation dose is discussed. Leaching from (Gd_1.4Ce_0.2La_0.1Sm_0.1Eu_0.1)(Zr_0.9Ti_0.9Sc_0.1In_0.1)O₇ was investigated. This composition facilitated comparison of numerous elemental dissolution rates from a single sample. Experimental conditions mimicked those likely in a repository at 1000m depth. Alteration of the solid was characterised by light and electron microscopy. The dissolved concentrations of leached elements were determined by inductively coupled plasma-mass spectrometry. Various synthetic routes were considered, to maximise the sample density.

When a ceramic laminate consisting of alternate dielectric and conducting layers is loaded, the dimensions of the laminae will change, leading to a corresponding change in capacitance across the dielectric layers. The effect of mechanical strain on the capacitance of individual dielectric layers was studied by measuring the capacitance changes in the outer layer of Al<SUB>2</SUB>O<SUB>3</SUB>/Pt and TiO<SUB>2</SUB>/Pt laminates loaded in three point bending. Metal conducting layers were used in this work to provide a simple conductor at room temperature, which would be compatible with both ceramic materials. It is shown that in both materials systems the measured capacitance at zero load agrees well with the predicted values for a parallel plate capacitor, and that on loading the capacitance varies linearly with strain. The rate of change of capacitance with strain agrees well with values predicted from dimensional changes in the dielectric layer, and is observed to change sign when the sign of the strain is reversed. The change in capacitance is also observed when the laminates are unloaded and reloaded, and it is shown that the rate of change of capacitance is independent of the loading rate. The measured capacitance is found to be a sensitive function of the a.c. frequency used in the measurement circuit, and it is shown that this is due to the presence of inductance in the circuit which originates from the connecting wires. The rate of change of capacitance with strain is also found to vary significantly with the a.c. frequency. This results from a variation of inductance with strain which is thought to be due to the small movement of the connecting wires when the laminate to which they are attached is deflected in bending. This necessitates the use of low a.c. frequencies at which inductance effects are negligible. The temperature of the dielectric is also found to influence the measured capacitance significantly, and it is shown that in A1<SUB>2</SUB>O<SUB>3</SUB> this can be overcome by comparing the capacitance of the strained body with another at the same temperature under zero strain. However this technique cannot be used in TiO<SUB>2</SUB> because of the large temperature coefficient of capacitance of this material.

Immersion tube heating systems are being used in some aluminium foundries, as they are more compact and thermally efficient than conventional radiative heating. Ceramic immersion tubes have been applied to these systems. However, the tubes can fail prematurely and unpredictably. This is thought to be due to the effects of the oxidising environment in which the tubes operate, and reaction with the molten aluminium alloys. The work described in this dissertation investigates these two factors. The ceramic in use is a Silicon Nitride-Bonded Silicon Carbide, SNBSC (particles of silicon carbide, SiC, in a porous silicon nitride, Si<SUB>3</SUB>N<SUB>4</SUB>, matrix), formed by a reaction bonding process. Four grades of this material, made using different quantities of fine and coarse SiC particles and differing sizes of silicon powder, were studied. The material was characterised using X-ray diffraction and optical, scanning electron and transmission electron microscopy. Dense (sintered) silicon carbide and silicon nitride ceramics were also studied to compare their behaviour with that of SNBSC. Oxidation was carried out in dry air at 1000°C, 1200°C and 1400°C. Thermogravimetric analysis (TGA) techniques were used to determine weight gains. Oxidised samples were studied using SEM and XRD. Changes in pore structure and surface chemistry were investigated by mercury porosimetry and Fourier Transform Infrared spectroscopy respectively. Oxidation was found to occur predominantly in the porous nitride phase, giving silica (SiO<SUB>2</SUB>) as crystalline cristobalite and a glassy silicate. The weight gain results can be fitted to an asymptotic law representing a process where the pores are progressively blocked by oxide product: Δm = Δm<SUB>∞</SUB>[1 - e<SUP>-k</SUP>a<SUP>t</SUP>] The kinetic results were in accord with measured changes in the pore structure. SNBSC with fine SiC oxidised more than the standard ceramic. Grades of SNBSC with coarse SiC had better oxidation resistance. Dense Si<SUB>3</SUB>N<SUB>4</SUB> oxidised parabolically, forming tridymite and silicates arising from sintering aids. At 750°C, liquid aluminium reacted slightly with unoxidised SNBSC if the alloy's silicon content was less than 5at%. Aluminium alloys readily wetted and reacted with oxidised material, and thus penetrated the pore network and enlarged pores. Interactions between aluminium and dense ceramics were in accord with results for SNBSC. Aluminium reacted with silica, producing alumina. This followed parabolic kinetics after an incubation period of approximately 4 minutes.

Many ceramic processing routes, such as tape casting, screen-printing and ink jet printing, involve deposition of a suspension of ceramic particles in a prescribed shape on a substrate. As the liquid evaporates from the suspension, the particles pack together and liquid leaves the pores. In water-based suspensions, these processes often occur at fronts that move from the edge to the centre of the drying layer. This is known as lateral drying. As drying proceeds, the body can change shape, residual stresses can develop and cracking can occur. However, existing models cannot predict either the shape changes that occur or adequately explain why the drying layer cracks. To study this, the drying behaviour of layers of alumina suspensions was investigated experimentally. Measurements were made of the movement of lateral fronts, the changes in local thickness and the activity at the surface of the layers, and the cracking behaviour and the dried thickness profiles were studied. By considering the redistribution of liquid at the first lateral front, where particles pack together, an expression for the thickness profile of the dried layers has been derived. Mass loss data, together with the local thickness measurements, suggest that the particles do not come into touching contact at this first front. Instead, a collapse event occurs when the repulsive layers between the particles are overcome. The development of stress in a drying layer is generally attributed to the build-up of capillary pressure at the surface of the body. However, this causes compression of the particle network, despite the constraint to shrinkage imposed by the substrate, and therefore cannot explain why cracking occurs. Here, it is proposed that the driving force for cracking actually arises from the collapse event. This predicts that cracking should occur while the space between the particles is still filled with liquid. This is consistent with the measurements of surface activity and local thickness, and the cracking behaviour observed.

Coal gasification poses many problems to the materials engineer due to the high temperatures and corrosive gases involved in the process. The main aim of this research is to investigate the influence of a simulated coal gasification environment on the mechanical properties and surface chemistry of a clay bonded SiC filter. This type of material is presently used to filter combustion gases and it is proposed that it may be used in gasification systems between the gasifier and gas turbine to filter particulates that would damage the turbine blades and lining. Three separate gases are used to investigate the corrosive effects of low pO<SUB>2</SUB>/high pS<SUB>2</SUB>, high pHC1 and increases in water vapour. Corrosion tests are performed at 1273K/1073K/873K at exposure times of 50, 100 and 200 hours. All physical and chemical observations are supported by theoretical modelling. The filter material is first characterised and found to be composed of SiC particles held together in an aluminosilicate clay binder. The only crystalline material identified in the clay is mullite. The 3-point fracture strength of the filter decreases substantially with increases in exposure time and temperature. It is concluded that the fluxing effects of the sodium present in the clay substantially lower its softening temperature. As a consequence, at temperatures of 1073K and above the filter is above its working temperature. Massive reductions in fracture strength of up to 40% seem to reflect this. The cause of the strength loss is believed to be due to both micro-cracking in the clay bond associated with the differential cooling rates of the clay and SiC, and mass loss from the clay bond due to volatile gas formation. The composition of the gas is found to significantly effect the chemistry of the surface reactions.

No description available.

No description available.